Pulsed-laser systems and methods for producing holographic stereograms

ABSTRACT

Pre-sensitization techniques can be used in conjunction with holographic recording materials to allow high quality holographic stereograms to be recorded in those holographic recording materials using pulsed lasers. Additional hologram production system hardware and software designs for use with pulsed lasers are disclosed.

[0001] This application claims the benefit, under 35 U.S.C. § 119 (e),of U.S. Provisional Application No. 60/334,834, filed Nov. 30, 2001,entitled “Pulsed-Laser Systems And Methods For Producing HolographicStereograms,” and naming Craig Newswanger, Pankaj Lad, Robert L. Sitton,Qiang Huang, Michael A. Klug, and Mark E. Holzbach as inventors; and ofU.S. Provisional Application No. 60/352,395, filed Jan. 28, 2002,entitled “Pulsed-Laser Systems And Methods For Producing HolographicStereograms,” and naming Craig Newswanger, Pankaj Lad, Robert L. Sitton,Qiang Huang, Michael A. Klug, and Mark E. Holzbach as inventors. Theabove-referenced applications are hereby incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates in general to the field of hologramproduction and, more particularly, to hologram production using pulsedlasers.

[0004] 2. Description of the Related Art

[0005] One-step hologram (including holographic stereogram) productiontechnology has been used to satisfactorily record holograms inholographic recording materials without the traditional step of creatingpreliminary holograms. Both computer image holograms and non-computerimage holograms can be produced by such one-step technology. In someone-step systems, computer processed images of objects or computermodels of objects allow the respective system to build a hologram from anumber of contiguous, small, elemental pieces known as elementalholograms or hogels. To record each hogel on holographic recordingmaterial, an object beam is typically directed through a spatial lightmodulator (SLM) displaying a rendered image and then interfered with areference beam. Examples of techniques for one-step hologram productioncan be found in the U.S. Pat. No. 6,330,088 entitled “Method andApparatus for Recording One-Step, Full-Color, Full-Parallax, HolographicStereograms,” Ser. No. 09/098,581, naming Michael A. Klug, Mark E.Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17,1998, (“the '581 application”) which is hereby incorporated by referenceherein in its entirety.

[0006] In general, the hologram production devices (often referred to as“hologram recorders”) described in the '581 application can use eithercontinuous-wave (CW) or pulsed lasers as the coherent light source forthe object and reference beams used to create interference patterns.Hologram recorders often use CW lasers because they are morecommercially available, their output intensity is typically easier tocontrol, and because it is typically easier to find a CW laser that willproduce output at a single desired frequency. Moreover, many of thepreferred holographic recording materials, such as photopolymerizablecompositions, dichromated gelatin, and silver halide emulsions, areparticularly suited for use with CW laser sources.

[0007] Nevertheless, the use of CW lasers in hologram recorders doespresent certain limitations. Chief among those limitations is therelatively low output power of CW lasers which causes the hologramrecorder to use relatively long exposure times (e.g., tens ofmilliseconds) for each hogel. During those exposure times, the entirehologram production system is particularly susceptible to mechanicalvibration. Great effort is expended to reduce or eliminate themechanical vibrations. Hologram recorders are typically located far awayfrom sources of environmental vibration, such as outside traffic,building vibration, mechanical equipment, common appliances, humanmotion, acoustic noise, plumbing turbulence and air flow. Specialdevices, such as vibrationally-isolated optics tables, are typicallyused where environmental vibration sources cannot be sufficientlyreduced or eliminated. Such devices and techniques add cost andcomplexity to hologram production systems. Moreover, to help ensure astable hogel recording environment, a step-repeat approach is oftenadopted in translating the holographic recording medium. Consequently,additional settling time (on the order of tens of milliseconds as well)is introduced into the recording process. The cumulative recording andsettling times prolong the hologram production process, making it moreexpensive and in some cases impractical for certain applications.Moreover, the mechanical systems used to step the system, bring (orallow) the system to come to a stop, and repeat can be very complex.

[0008] Using pulsed lasers in hologram production devices can mitigateor solve many of the aforementioned problems associated with CW laseruse. Due to the different physics of pulsed laser operation, a smallframe pulsed laser is able to generate higher light intensity than itsCW counterparts. For example, small frame frequency doubled Nd:YAGpulsed lasers can generate 1.1 mJ of energy during a 35 ns long pulse at532 nm. This corresponds to approximately 31.4 kW of power during thepulse. In contrast, a typical CW Nd:YAG laser produces approximately 4 Wof power. Because high exposure intensity is possible using pulsedlasers, the required exposure time to generate a hologram can be reducedsignificantly. While tens of milliseconds is typically needed for CWlaser hologram recording, the tens of nanoseconds pulse duration of apulsed laser is adequate for pulsed laser hologram recording. Decreasingthe exposure time by six orders of magnitude means that the frequenciesof both the movement of the hologram recorder components andenvironmental vibration are too low to generate any noticeable effect oninterference pattern generation. The mechanical stability requirementsrestricting the CW laser based hologram recorder are completely relaxed.Consequently, the recorder design can be significantly simplified andthe cost of the hardware is reduced.

[0009] Despite the advantages of using pulsed lasers in hologramproduction devices, the holographic recording materials typically usedmay not provide adequate results when used with pulsed lasers. Forexample, photopolymerizable compositions (photopolymers) are among themost preferable holographic recording materials because of the imagerecording capabilities and their relative ease of use. Photopolymersinclude a wide range of materials that undergo physical, chemical, oroptical changes through selective polymerization when exposed to light.Typically, photopolymers include a monomer or a crosslinkable polymer, asensitizer or photoinitiator, and a binder or liquid to hold thecomponents. Changes in the photopolymer's refractive index,transparency, adhesion, and/or solubility differentiate light and darkregions when these materials are exposed to an activating light source.Photopolymers capable of recording volume phase holograms include thosedeveloped by Canon Incorporated (based on polyvinyl carbazole), PolaroidCorporation (based on polyethylene amine/acrylate), and E. I. du Pont deNemours and Company (based on polyvinyl acetate and polymethylmethacrylate). Those having ordinary skill in the art will readilyrecognize that a variety of different photopolymer compositions can beused in the practice of the inventions described herein. Nevertheless,preferred photopolymer films are provided by E. I. du Pont de Nemoursand Company under the trade designations, for example, OmniDex™ 706,OmniDex™ 801, HRF-800X001-15, HRF-750X, HRF-700X, HRF-600X, and thelike.

[0010] Holograms recorded in photopolymer films using single laserpulses from pulsed lasers are known to be of generally poorer quality ascompared to holograms recorded in photopolymer films using CW lasers.For example, in V. N. Mikhailov, K. T. Weitzel, V. N. Krylov, and Urs P.Wild, “Pulse Hologram Recording in DuPont's Photopolymer Films,”Practical Holography XI, Proc. SPIE, vol. 3011, pages 200-202, Feb.10-11, 1997, (the Mikhailov reference) which is hereby incorporated byreference herein in its entirety, it was demonstrated that a hologramrecorded with a 25 ns pulse from a YLF-Nd Q-switched laser (0.25 J/cm²intensity) had a peak diffraction efficiency of approximately 6.5%,while a hologram recorded for 5 seconds using a comparable intensityargon-ion CW laser had a peak diffraction efficiency of approximately92%. Diffraction efficiency is a typical measurement of the quality of arecorded hologram and is based on the ratio of diffracted lightintensity to input light intensity (usually neglecting Fresnelreflection and absorption in the holographic recording material).

[0011] The Mikhailov reference goes on to demonstrate that hologramswith larger diffraction efficiencies can be recorded using pulsed lasersif the photopolymer film is pre-illuminated. Specifically, the Mikhailovreference demonstrates that pulsed laser recorded holograms can havediffraction efficiencies of approximately 40% and 75% when thephotopolymer film is pre-illuminated using a pulse from the pulsed laserand filtered incoherent light, respectively.

[0012] Accordingly, it is desirable to have improved systems and methodsfor using pulsed lasers to produce holograms and particularlyholographic stereograms. Such improved systems and methods would providehigh-quality recorded holograms while allowing the hologram productionsystems to take full advantage of the use of pulsed lasers.

SUMMARY OF THE INVENTION

[0013] It has been discovered that pre-sensitization techniques can beused in conjunction with holographic recording materials to allow highquality holographic stereograms to be recorded in those holographicrecording materials using pulsed lasers. Additional hologram productionsystem hardware and software designs for use with pulsed lasers aredisclosed.

[0014] Accordingly, one aspect of the present invention provides amethod of recording holograms. A holographic recording material isprovided. The holographic recording material is pre-sensitized byexposing the holographic recording material to an incoherent broadbandlight source. The holographic recording material is exposed to aninterference patterned formed by a reference beam from a pulsed laserand an object beam from the pulsed laser.

[0015] Another aspect of the present invention provides a method ofrecording holograms. A holographic recording material is provided. Theholographic recording material is pre-sensitized by exposing theholographic recording material to a plurality of laser pulses. Theholographic recording material is exposed to an interference patternedformed by a reference beam from a pulsed laser and an object beam fromthe pulsed laser.

[0016] Still another aspect of the present invention provides a methodof recording holograms. The holographic recording material ispre-sensitized by exposing the holographic recording material to atleast one of an incoherent broadband light source and a plurality oflaser pulses. At least one of a reference beam from a pulsed laser andan object beam from the pulsed laser is oriented at an oblique anglewith respect to the holographic recording material. The holographicrecording material is exposed to an interference pattern formed by thereference beam and the object beam.

[0017] Yet another aspect of the present invention provides an apparatusfor recording holographic stereograms including a pulsed light sourcethat produces a coherent beam, a material holder, and an optical system.The material holder holds a pre-sensitized holographic recordingmaterial, the pre-sensitized holographic recording material beingpre-sensitized by exposing the holographic recording material to atleast one of an incoherent broadband light source and a plurality oflaser pulses. The optical system is operable to direct at least aportion of the coherent beam to the holographic recording material.

[0018] The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. As willalso be apparent to one of skill in the art, the operations disclosedherein may be implemented in a number of ways, and such changes andmodifications may be made without departing from this invention and itsbroader aspects. Other aspects, inventive features, and advantages ofthe present invention, as defined solely by the claims, will becomeapparent in the non-limiting detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present inventions may be better understood, and theirnumerous objects, features, and advantages made apparent to thoseskilled in the art by referencing the accompanying drawings.

[0020]FIG. 1 is a schematic diagram a hologram production device using apulsed laser.

[0021]FIG. 1A is a schematic diagram of a color module that can be usedwith the hologram production device shown in FIG. 1.

[0022]FIG. 2 illustrates an optical path length matching device for usein a hologram production device such as that shown in FIG. 1.

[0023]FIG. 3 illustrates an object-beam/reference-beam interferometerfor use in a hologram production device such as that shown in FIG. 1.

[0024]FIG. 4 is a schematic diagram a horizontal-parallax-only hologramproduction device using a pulsed laser.

[0025]FIG. 5 illustrates an example of interlaced hologram production.

[0026]FIGS. 6A and 6B illustrate two different techniques for providinga reference beam in a horizontal-parallax-only hologram productiondevice.

[0027]FIG. 7 is a graph showing the motion profile of a lineartranslator suitable for positioning holographic recording material.

[0028]FIGS. 8A and 8B illustrate correct and incorrect positioning ofthe holographic recording material used in a hologram production device.

[0029]FIGS. 9A and 9B illustrate correct and incorrect image displaysynchronization with the positioning of the holographic recordingmaterial used in a hologram production device.

[0030]FIG. 10 shows a graph including both the motion profile of alinear translator suitable for positioning holographic recordingmaterial and a video frame synchronization signal.

[0031]FIG. 11 shows a graph similar to that of FIG. 10 but where thehorizontal axis is shown in units with respect to another video framesynchronization signal.

DETAILED DESCRIPTION

[0032] The following sets forth a detailed description of the bestcontemplated mode for carrying out the invention. The description isintended to be illustrative of the invention and should not be taken tobe limiting.

[0033] The Mikhailov reference and related references such as V. N.Mikhailov, K. T. Weitzel, T. Y. Latychevskaia, V. N. Krylov, and Urs P.Wild, “Pulse Recording of Slanted Fringe Holograms in DuPontPhotopolymer,” Holographic Materials IV, Proc. SPIE, vol. 3294, pages132-135, March 1998, which is hereby incorporated by reference herein inits entirety, disclose that the pulsed laser object beam and referencebeam used to record holograms in pre-illuminated photopolymer films arecounterpropagating beams directed at either a 0° or 12° angle ofincidence (i.e., measured with respect to the surface normal of theholographic recording material). However, the applicants have discoveredthat a significant increase in diffraction efficiency of such recordedholograms occurs where the angle of incidence of at least one of therecording beams is oblique, i.e., where the angle of incidence isgreater than 12°.

[0034] This is illustrated in FIG. 1, where the angle of incidence ofthe reference beam in one embodiment of a pulsed laser hologramrecording system is approximately 45°. The applicants found that such arecording geometry benefited both pulsed laser hologram recording(particularly pulsed laser hologram recording where the holographicrecording material is pre-sensitized as discussed below) and CW laserrecording.

[0035] The Mikhailov reference discloses two types of pre-illuminationthat are used to pre-sensitize the photopolymer film prior to recordingholograms with a pulsed laser: single laser pulse pre-sensitization andincoherent light pre-sensitization using a green filter. The applicantshave developed additional pre-sensitization and hologram recordingtechniques that provide results superior to those disclosed in theMikhailov reference. Additionally, the work of Mikhailov et al.,demonstrated the effectiveness of their pre-illumination techniques forrecording interference patterns with no significant amplitude orintensity modulation of the object beam (i.e., the object beam wasmerely reflected from a mirror). The applicants, however, havedemonstrated the recording of diffuse grating holograms (e.g., using anobject beam intensity modulated by a spatial light modulator) using thedisclosed devices and techniques.

[0036] Incoherent broadband pre-illumination light sources can be usedregardless of the wavelength at which the hologram will be recorded. Forexample, unfiltered halogen, florescent, incandescent, and white-lightlight emitting diode (LED) light sources can be used to pre-sensitizethe holographic recording material. Additionally, narrowband incoherentlight sources such as various color LEDs can be used alone or incombination (e.g., a combination of red, green, and blue LEDs) toeffectively produce a sufficiently broadband light source.

[0037] Pre-illumination of the holographic recording material can beconducted in a scanning fashion, where only a portion of the holographicrecording material is illuminated at any one time. Such apre-illumination technique is particularly suited for hologramproduction devices designed to utilize a continuous supply ofphotopolymer material or “web.” For example, if the photopolymer web issupplied from a roll of film on one roller and taken-up by a secondroller, a stationary light source positioned above the film canpre-sensitize the film as it passes by the light source. Alternately, alight source mounted on a motion control stage or arm can be used toscan the holographic recording material.

[0038] The holographic recording material can also be flood illuminatedwhere the entire holographic recording material is simultaneouslyilluminated. This is particularly suited for hologram recorders designedto record on single tiles or sheets of holographic recording material.Either as part of the hologram recorder, or as part of a separatepre-sensitization stage, a light source can be located above a sheet ofthe holographic recording material and activated to flood illuminate theholographic recording material. In such examples, the holographicrecording material is typically placed on or laminated to a glass orplastic substrate before the holographic recording material ispre-sensitized. The holographic recording material typically remains onthe substrate while the desired holograms are recorded in the material,i.e., after pre-sensitization. In the case of the aforementioned Du Pontphotopolymer films, laminating the film to the substrate typicallyinvolves removing the film's cover sheet and placing the tackyphotopolymer film on the substrate surface.

[0039] With any type of pre-illumination, the amount ofpre-sensitization can be controlled by adjusting the intensity of thelight source, the holographic recording material's exposure time to thelight source, or some combination of the two. Pre-illumination energy ofapproximately 2 mJ/cm² has been successfully used in conjunction withthe aforementioned DuPont photopolymers, particularly the OmniDex™ 801and HRF-700X photopolymers. The applicants have discovered that thepre-sensitization effect does not, in general, decrease with time.Pre-illumination can be conducted a few seconds before hologramrecording or hours before hologram recording with no noticeablevariation in results. In one example, five hours lapsed betweenpre-illumination and hologram recording with no noticeable effect onhologram quality. Pre-illumination can also be conducted with a pulsedlaser source, e.g., the pulsed laser that is used to create the objectand reference beams, using multiple pulses to pre-sensitize theholographic recording material. Moreover, the applicants have discoveredthat the amount of energy required for adequate pre-sensitization of thephotopolymer films can vary depending the wavelength used and/or thewavelength used to record a hologram recorded in the pre-sensitizedfilm. For example, the energy needed for blue pre-sensitization has beenfound to be approximately half that needed for green pre-sensitization.

[0040] High diffraction efficiency holograms can also be recorded usingmultiple pulse exposure recording. In one example, one portion of theholographic recording material is exposed to the same pulsedlaser-created interference pattern multiple times. While it is importantto maintain the same optical geometry setup (including the renderedcomputer graphics image that is typically used to modulate the objectbeam) from one pulse exposure to the next, pulse-to-pulse coherence isnot required. Thus, lower energy pulsed lasers (or lower energy pulsesfrom a given pulsed laser) can be used by providing multiple pulseexposure. The highest diffraction efficiencies are achieved where thetime between pulses used for multiple pulse exposure is not too short.For example, in an experiment where holograms were recorded in a 25 mm²area using six 35 ns pulses of 0.5 mJ/cm², 100 Hz, 1 kHz, and 10 kHzpulse frequencies produced diffraction efficiencies of 89%, 83%, and 12%respectively. Additionally, the multiple pulse exposures need not usethe same laser-created interference pattern for each exposure. In anexample where the object beam of the system is modulated by an SLM, thefirst exposure can be created using a “white” image (e.g. a white screenas displayed on the SLM) while the second exposure can be created usingthe desired image.

[0041] Experiments have shown that typical photopolymers have differentresponses to laser pulse exposure and CW laser exposure. In general forCW laser exposure, recorded holograms show maximum diffractionefficiency when the photopolymer is saturated, and overexposure does notdecrease the diffraction efficiency of recorded holograms. However,photopolymers can exhibit a decrease in the diffraction efficiency ofrecorded holograms if the photopolymer is overexposed with one or morelaser pulses.

[0042] Tables 1-3 show the results of several other experiments. In eachtable, the diffraction efficiency of the pulsed laser recorded hologramsis shown. For each of the experiments, a pulsed frequency-doubled Nd:YAGlaser (532 nm) was used to create an interference pattern that wasrecorded in a sample of DuPont OmniDex™ 801 photopolymer film, thesample area exposed was 5 mm by 5 mm (Tables 1 and 2) and 10 mm by 10 mm(Table 3), the pulse length was 35 ns, and reference beam was incidenton the holographic recording material at an angle of 45° with respect tothe normal.

[0043] Table 1 shows the results from single pulse exposure experimentswhere the holographic recording material was pre-illuminated for 5 swith a broadband (e.g., white light) incoherent light source having anintensity of 0.5 mW/cm². Diffraction efficiency in excess of 90% wasobtained. Experiments with the amount of pre-illumination using ahalogen white-light showed best results where the total amount ofpre-illumination energy was approximately 2.5 mJ/cm² (e.g.,approximately 6 seconds exposure to a halogen source having an intensityof 0.4 mW/cm²). Diffraction efficiency of recorded holograms tended todecrease when the pre-illumination energy was less than or greater than2.5 mJ/cm². TABLE 1 Pulse Energy per Sample Pulse Energy per cm²Efficiency (mJ/Sample) (mJ/cm²) (%) 0.1 0.4 83 0.2 0.8 80 0.3 1.2 67 0.41.6 87 0.5 2.0 84 0.6 2.4 85 0.7 2.8 85 0.8 3.2 87 0.9 3.6 87 0.95 3.891

[0044] Table 2 shows the results from multiple pulse exposureexperiments where the holographic recording material was pre-illuminatedfor 5 s with a broadband (e.g., white light) incoherent light sourcehaving an intensity of 0.5 mW/cm². Each pulse had an energy of 0.1 mJ,and the pulses were repeated by manual triggering. Diffractionefficiencies in excess of 90% were obtained. TABLE 2 Total Energy percm² Efficiency Number of Pulses (mJ/cm²) (%) 1 0.4 61 2 0.8 82 3 1.2 904 1.6 87 5 2.0 89 6 2.4 90 7 2.8 92 8 3.2 92 9 3.6 92 10 4.0 93

[0045] As noted above, over exposure of the photopolymer tended toreduce the diffraction efficiency of recorded holograms. For example,cumulative exposure energies of 8-10 mJ/cm² tended to reduce diffractionefficiency of the recorded hologram to approximately 87-88%.

[0046] Table 3 shows the results from multiple pulse exposureexperiments where the holographic recording material was notpre-illuminated. Each pulse had an energy of 0.5 mJ, and the pulses wererepeated by manual triggering. Diffraction efficiencies approaching 100%were obtained. TABLE 3 Total Energy per cm² Efficiency Number of Pulses(mJ/cm²) (%) 3 1.5 61 6 3.0 96 9 4.5 98 12 6.0 99 15 7.5 99

[0047] Experiments with red (628 nm) and blue (443 nm) laser pulses showthat pre-sensitization of the photopolymer is also effective for thesewavelengths as well. In one example, the photopolymer was pre-sensitizedusing white light and energy thresholds (cumulative laser pulse energy)for hologram recording were 12 mJ/cm² for red laser pulses and 5 mJ/cm²for blue laser pulses.

[0048]FIG. 1 is a schematic diagram a pulsed laser based hologramproduction device that can take advantage of the above-describedpre-sensitization and recording techniques. Hologram recorder 100 isdesigned to rapidly produce horizontal-parallax-only (HPO) or fullparallax holograms and particularly holographic stereograms. The maincomponents of hologram recorder 100 are pulsed laser 110, synchronizedholographic recording material translating system 120, object beamoptical system 130 and reference beam optical system 140. Althoughhologram recorder 100 is shown having only one pulsed laser, hologramrecorder 100 can in general be constructed with several different pulsedlasers (or one pulsed laser capable of providing laser output atmultiple wavelengths) to enable recording of multi-color holograms andholographic stereograms. Thus, the systems and methods described in theaforementioned '581 application can be extended to the use of pulsedlaser hologram recorders such as recorder 100

[0049] An example of a pulsed laser 110 that can be used in hologramrecorder 100 is the 212 Series (e.g., model 212S-532-3500) diode-pumped,Q-switched pulsed Nd:YAG laser manufactured by Lightwave ElectronicsCorporation of Mountain View, Calif. Those having ordinary skill in theart will readily recognize that a variety of different pulsed lasers canbe used in hologram recorder 100 In general, the laser wavelength, laserpulse power, and laser pulse energy of a particular pulsed laser systemare the determining factors for use in a hologram recorder.

[0050] For multiple color, e.g., red-green-blue (RGB), laser pulses, avariety of different laser systems can be used including diode-pumpedsolid state lasers, flash-lamp pumped solid state lasers, and dyelasers. Typical solid-state laser gain media include ruby, sapphire,garnet, alexandrite, Titanium sapphire (Ti:sapphire), Neodimium:YttriumAluminum Garnet (Nd:YAG), and Neodimium:Yttrium Lithium Fluoride(Nd:YLF). In one example, optical parametric oscillators (OPOs) are usedto down convert laser frequency. For example, a frequency tripled Nd:YAGlaser can produce 355 nm pulses which in turn drive a tunable OPO toproduce pulses ranging from 410 nm to 690 nm. In another example, aNd:YLF laser produces 1047 nm pulses which are then converted throughsecond-harmonic generation to 523 nm pulses used to drive an OPO. Outputfrom the OPO at 898 nm and 1256 nm can be frequency doubled throughsecond harmonic generation to yield 449 nm and 628 nm pulsesrespectively. In another example, Raman converters can be utilized. Theoutput of a pulsed alexandrite laser (e.g., 764 nm pulses) is frequencydoubled through second harmonic generation to yield 382 nm pulses. Thesepulses then pass through a Raman cell including Deuterium Hydride (HD)gas. Careful selection of the input pulse can yield, for example, outputlaser pulse of 443 nm, 527 nm, and 650 nm. Other types of pump lasers, eg., Nd:YAG and Nd:YLF, and other gases for the Raman cell, e.g.,Deuterium (D₂) or methane (CH₄), can be used. Moreover, some combinationof all or some of these techniques and lasers can be used to produce thedesired pulse wavelengths.

[0051] The pulsed laser beam produced by pulsed laser 110 is split intoobject and reference beams by the beam splitter C1, typically apolarizing beamsplitter cube. The polarizations and relative intensitiesof the object and reference beams (i.e., the beam ratio) are controlledby retarders P1 and P2, typically half-wave plates.

[0052] Because holographic recording materials typically have differentsensitivities to different laser wavelengths, using multiple color laserpulses may require use of a color balancing device such as color module150 as shown in FIG. 1A. Color module 150 typically receives a multiplecolor beam 155, referred to generally as a “white” beam. Dispersingprism 160 separates the incoming multiple color beam into itsconstituent colors. Each beam is then reflected by its associated mirror165A, 165B, or 165C. Retarders 170A, 170B, or 170C, typically half-waveplates, in conjunction with polarizing beam splitters 175A, 175B, or175C are used to adjust the respective beam's intensities. Thus theappropriate beam intensity for each color can be achieved. Excess beamenergy is directed to beam dumps 180A, 180B, or 180C. The various beamsare recombined using dichroic mirrors 185A, 185B, or 185C. The balancedoutput beam 190 can then be introduced into the remainder of hologramrecorder 100

[0053] The object beam is then expanded and collimated by a collimatorformed through the combination of lenses L1 and L2. Next, the objectbeam is reflected by beamsplitter cube C2 into spatial light modulator(SLM) SLM where the object beam wavefront is intensity modulated.Spatial light modulator SLM as illustrated is a reflective SLM whichrotates the polarization state of the object beam. In general, a varietyof different SLMs can be used including, but not limited to, atransmissive LCD panel, a reflective LCD panel, an optically addressedLCD panel, a digital micro-mirror array, film, or a transparency. TheSLM typically receives image input via a video cable from a computersystem (not shown). Additionally, multiple SLMs can be used havingimages generated in parallel by multiple central processing units orcomputer systems. Thus, the response time of the SLM is typically animportant parameter. Examples of SLMs for use in hologram recorder 100include the Digital Direct Drive Image Light Amplifier (D-ILA®) seriesof reflective LCD devices manufactured by the Victor Company of Japan,Ltd. (JVC). Additionally, a single multiple color SLM can be used, ormultiple SLMs can be used (typically one SLM for each beam color). Theimages displayed on the SLM, and thus the images used to intensitymodulate the object beam, are typically computer graphics images (eitherrendered or converted images of real objects) designed and/or processedfor recording as holograms.

[0054] The modulated wavefront is relayed and filtered by the lens pairL3 and L4 and aperture A1 to then form an image on a band-limiteddiffuser or an anisotropic diffuser BLD. Note that, in general, thediffuser can be located in a variety of different positions throughoutthe system. The image then passes through a Fourier transform lens FTLthereby generating the desired object beam wave front at the holographicrecording material positioned on recording plate RP. Note that althoughhologram recorder 100 is shown using lens pair L3 and L4, to, forexample, remove undesired effects such as, but not limited to, highfrequency image components such as pixel or grid artifacts that resultedfrom an SLM display with pixels, a variety of different optical systemscan be used.

[0055] In reference beam optical system 140, the reference beam istransmitted through path length matching optics (mirrors M1, M2, M3, andM4) and illuminates the reference beam-shaping aperture A2. Path lengthmatching optics are used to adjust the path length of the reference beampath to match that of the object beam, or to at least bring the twopaths within a distance of each other that is less than or equal to thecoherence length of pulsed laser 110. For some pulsed lasers, thecoherence length can be on the order of several millimeters. The imageof shaping aperture A2 is then relayed via reference beam relay opticsL5 and L6 to the holographic recording material at recording plate RP.As shown, the angle of incidence of the reference beam with respect tothe surface normal of the holographic recording material at recordingplate RP is preferably oblique, and further preferably approximates 45°.In other examples, the angle of incidence of the reference beam withrespect to the surface normal of the holographic recording material isapproximately 0°. A variety of different techniques can be used steereither or both of the reference beam and the object beam. For example,the devices and techniques of the aforementioned '581 application can beutilized. Finally, the object and reference beams are superimposed atthe holographic recording material on recording plate RP producing theinterference pattern required for hologram (or hogel) recording.

[0056] In the example of FIG. 1, the optics systems 130 and 140 aregenerally kept stationary during the operation of hologram recorder 100while the synchronized holographic recording material translating system120 is used to reposition the holographic film located at recordingplate RP for each hogel that is recorded. Synchronized holographicrecording material translating system 120 is typically a computercontrolled x-y translation system. In one example, synchronizedholographic recording material translating system 120 includes a 300ATseries positioning system manufactured by the Daedal Division (Irwin,Pa.) of the Parker Hannifin Corporation and an ACR2000 positioncontroller manufactured by Acroloop Motion Control Systems, Inc., ofChaska, Minn. The synchronization of holographic recording materialtranslation, SLM computer graphics image display, and laser pulsing isfurther described below in conjunction with FIGS. 4-8. Alternately, theoptics system can be designed to move or to provide the object andreference beams at varying locations as described, for example, in the'581 application.

[0057] It should be noted that it is well within the skill of one havingordinary skill in the art to substitute different optical components formany of the specific optical components shown in FIG. 1. For example, avariety of different polarizing devices, beam splitters, collimatingoptics, lenses, SLMs and mirrors can be used in hologram recorder 100Additionally, although FIG. 1 illustrates a system for producingreflection holograms, systems for producing transmission holograms usingthe devices and techniques described above can also be implemented.

[0058]FIG. 2 illustrates an optical path length matching device 200 foruse in a hologram recorder 100 Optical path length matching device 200is a more detailed example of path length matching optics, such asmirrors M1, M2, M3, and M4 of FIG. 1. In order to create a suitableinterference pattern using the object and reference beams, it isdesirable to maintain the coherence of the two laser beams. The temporalcoherence of a laser is often measured in terms of the laser's coherencelength, that is the distance the beam will travel during which itremains coherent. For many pulsed lasers the coherence length is only onthe order of several millimeters. If the difference in the path lengthsof the object and reference beams is greater than the laser's coherencelength, the two beams will no longer be coherent and an adequateinterference pattern cannot be formed. Optical path length matchingdevice 200 allows hologram recorder 100 to use a pulsed laser with asmall coherence length yet still achieve adequate hologram recording.

[0059] The reference beam is received from pulsed laser 110 at mirror M1which reflects the beam to mirror M2. Mirror M1 is typically located ata fixed position in optical path length matching device 200. Mirrors M2and M3 are mounted together on a movable carrier 220 which is allowed toslide along a straight rail 210. Because of the straightness andrigidity of rail 210, the moving path of the carrier remains parallel tothe beam path from mirror M1 to mirror M2 and from mirror M3 to mirrorM4. From mirror M4, the reference beam is reflected toward referencebeam shaping aperture A2 and on toward the holographic recordingmaterial at recording plate RP. Moving the carrier varies the totaloptical path of reference beam optical system 140 while maintaining beamalignment. Fine position adjustment (e.g., using an attached micrometer)of movable carrier 220 allows the path lengths of the object andreference beams, at least when measured at the respective centers of thebeams, to be within microns of each other. The position of movablecarrier 220 can be manually adjusted by a hologram recorder operatorvisualizing a sample interference pattern created using the object andreference beams or computer adjusted using an automatic feedback systemthat monitors fringe contrast in a sample interference pattern createdusing the object and reference beams.

[0060]FIG. 3 illustrates an example of an object-beam/reference-beaminterferometer 300 for use in hologram recorder 100 and preferably inconjunction with an optical path length matching device such as opticalpath length matching device 200. Object-beam/reference-beaminterferometer 300 allows an operator of hologram recorder 100 tovisualize the interference pattern generated by the superposition of theobject and reference beams. Half-mirrored beam combiner HM receives theobject beam from previously described Fourier transform lens FTL. Apattem-magnifying lens PML is positioned between half-mirrored beamcombiner HM and a projection screen 310 located in the far field. Whenobject-beam/reference-beam interferometer 300 is in use (e.g., inconjunction with the adjustment of the optical path length for thereference beam) half-mirrored beam combiner HM is placed at a pointwhere the object and reference beams overlap. The object beam(illustrated in FIG. 3 as solid lines) transmits through thehalf-mirrored beam combiner HM while the reference beam (illustrated inFIG. 3 as dashed lines) is reflected by half-mirrored beam combiner HM.By adjusting the angle of half-mirrored beam combiner HM, the object andreference beams can be aligned in front of pattem-magnifying lens PML toform a low frequency interference pattern. Pattern-magnifying lens PMLmagnifies interference pattern for display on screen 310 for analysis.

[0061] Analysis of the fringes can be conducted by a hologram recorderoperator. For example, while adjusting the position of movable carrier220 in optical path length matching device 200 the operator of hologramrecorder 100 can observe changes in the interference pattern formed bythe object and reference beams. The best optical path length match isindicated by the highest interference fringe contrast observed on screen310. Typically, a photodetector is used to measure the fringe contrastof the interference pattern either by examining the interference patternprojected on screen 310 or by being positioned in the place of screen310 to receive the interference pattern. With appropriate detectioncircuitry, the photodetector can provide a signal for use in adjustingthe optical pathlength of reference beam optical system 140.

[0062] In addition to assisting in the optical path length matchingprocess, object-beam/reference-beam interferometer 300 can also be usedto check hologram recorder 100 system polarization.Object-beam/reference-beam interferometer 300 can also be used tomonitor system stability including the presence of undesirablevibrations.

[0063] Although hologram recorder 100 of FIG. 1 can, in general, be usedto produce HPO holograms, certain system optimizations can be made toproduce a recorder more suitable for HPO holograms. As is well known inthe art, an HPO hologram does not contain any vertical parallaxinformation and thus its production uses significantly less data than afull-parallax hologram. Moreover, the recording time for an HPO hologramcan be significantly shorter than that for a full-parallax hologram. HPOholograms typically have a different hogel structure as compared tofull-parallax holograms. Instead of square hogels, or at least hogelshaving a roughly one-to-one aspect ratio, HPO holograms typically usestretched rectangular hogels having larger aspect ratios. The length ofan HPO hogel is usually equal to the entire vertical dimension of thehologram. In addition, recording HPO holograms typically requires filmtranslation (or alternately optics translation) in only one dimension.Due to these differences, a recording mechanism for HPO holograms can beimplemented in a number of different ways.

[0064]FIG. 4 illustrates one example of a pulsed laser based hologramproduction device optimized for production of HPO holograms and takingadvantage of the above-described pre-sensitization and recordingtechniques. Hologram recorder 400 is designed to rapidly produce HPOholograms and particularly holographic stereograms. The main componentsof hologram recorder 400 are pulsed laser 410 (generally similar topulsed laser 110 described above), synchronized holographic recordingmaterial translating system 420, object beam optical system 430 andreference beam optical system 440. Although hologram recorder 400 isshown having only one pulsed laser, hologram recorder 400 can in generalbe constructed with several different pulsed lasers (or one pulsed lasercapable of providing laser output at multiple wavelengths) to enablerecording of multi-color holograms and holographic stereograms. Thus,the systems and methods described in the aforementioned '581 applicationcan be extended to the use of pulsed laser hologram recorders such asrecorder 400.

[0065] The pulsed laser beam produced by pulsed laser 410 is split intoobject and reference beams by the beam splitter C1, typically apolarizing beamsplitter cube. The polarizations and relative intensitiesof the object and reference beams (i.e., the beam ratio) are controlledby retarders P1 and P2, typically half-wave plates. Because holographicrecording materials typically have different sensitivities to differentlaser wavelengths, using multiple color laser pulses may require use ofa color balancing device such as color module 150 as shown in FIG. 1Aand described above.

[0066] Referring to object beam optical system 430, the object beam isexpanded and collimated by a collimator formed through the combinationof lenses L1 and L2. Next, the object beam is reflected by beamsplittercube C2 into spatial light modulator (SLM) SLM where the object beamwavefront is intensity modulated. Spatial light modulator SLM asillustrated is a reflective SLM which rotates the polarization state ofthe object beam. In general, a variety of different SLMs can be usedincluding, but not limited to, a transmissive LCD panel, a reflectiveLCD panel, an optically addressed LCD panel, a digital micro-mirrorarray, film, a projection or a transparency. The SLM typically receivesimage input via a video cable from a computer system (not shown).Additionally, multiple SLMs can be used having images generated inparallel by multiple central processing units or computer systems. Thus,the response time of the SLM is typically an important parameter.Examples of SLMs for use in hologram recorder 400 include the DigitalDirect Drive Image Light Amplifier (D-ILA®) series of reflective LCDdevices manufactured by the Victor Company of Japan, Ltd. (JVC).Additionally, a single multiple color SLM can be used, or multiple SLMscan be used (typically one SLM for each beam color). The imagesdisplayed on the SLM, and thus the images used to intensity modulate theobject beam, are typically computer graphics images (either rendered orconverted images of real objects) designed and/or processed forrecording as holograms.

[0067] The modulated wavefront is relayed and filtered by the lens pairL3 and L4 and aperture A1 to then form an image on a band-limiteddiffuser or an anisotropic diffuser BLD′. Although BLD′ can be the sameas or similar to BLD of hologram recorder 100, BLD′ can also be a bandlimited diffuser designed specifically for HPO hologram production aswill be described below. Moreover, the diffuser can be located in avariety of different positions throughout the system. The image thenpasses through a Fourier-transform cylindrical lens FCL therebygenerating the desired object beam wave front and forming a “line”shaped hogel exposing area at the holographic recording materialpositioned on recording plate RP. Since FCL has power only in onedimension, it provides a one-dimensional view zone for the resultanthologram. This view zone is in the horizontal orientation of thehologram to provide the horizontal parallax. To broaden the verticalview zone for the hologram, additional optics denoted as the verticaldiffuser VD (described below) can be optionally inserted just before therecording plate RP. If it is desirable to keep the vertical view zonenarrow, vertical diffuser VD may not be needed. Alternately, verticaldiffuser VD can be located in a different portion of the system. Notethat although hologram recorder 400 is shown using lens pair L3 and L4,to, for example, remove undesired effects such as, but not limited to,high frequency image components such as pixel or grid artifacts thatresulted from an SLM display with pixels, a variety of different lenssystems can be used.

[0068] In reference beam optical system 440, the reference beam istransmitted through path length matching optics (mirrors M1, M2, M3, andM4 and as described in greater detail with respect to FIG. 2) andilluminates lens L5′. Path length matching optics are used to adjust thepath length of the reference beam path to match that of the object beam,or to at least bring the two paths within a length of each other that isless than or equal to the coherence length of pulsed laser 410. For somepulsed lasers, the coherence length can be on the order of severalmillimeters.

[0069] Next, the reference beam goes through two sets of beam shapingoptics. The lens pair L5′ and L6′ forms a telescope that correctsreference beam divergence and focuses the beam at recording plane RP.After this telescope, cylindrical lens pair CL1 and CL2 serves as aone-dimensional beam expander and collimator so that after lens CL2, thereference beam has the appropriate shape, e.g., a light “ribbon”. Thelength of the image should be long enough to cover the entire verticaldimension of the final hologram. The width of the image, which has aGaussian profile, is made very thin by adjusting the telescope formed byL5′ and L6′. Note that instead of cylindrical optics, other optics canbe used to produce the desired reference beam shape. For example, aPowell lens (a particular type of lens having an aspheric tip) can beused to generate a line of quasi-even intensity light. The image width,typically on the order of 100 microns, determines the hogel resolutionon the recording plane. After the beam shaping, the reference beam isfolded by M5′ and then deflected 45° by a diffraction grating GT beforeilluminating the recording plane. As shown, the angle of incidence ofthe reference beam with respect to the surface normal of the holographicrecording material at recording plate RP is preferably oblique, andfurther preferably approximates 45°. In other examples, the angle ofincidence of the reference beam with respect to the surface normal ofthe holographic recording material is approximately 0°. Finally, theobject and reference beams are superimposed at the holographic recordingmaterial on recording plate RP producing the interference patternrequired for hologram (or hogel) recording.

[0070] In the example of FIG. 4, the optics systems 430 and 440 aregenerally kept stationary during the operation of hologram recorder 400while the synchronized holographic recording material translating system420 is used to reposition the holographic film located at recordingplate RP for each hogel that is recorded. Synchronized holographicrecording material translating system 420 is generally similar totranslating system 120, but need only translate the holographicrecording material in one dimension. Examples of such translationsystems and their control mechanisms are described elsewhere in thisapplication. Alternately, the optics system can be designed to move orto provide the object and reference beams at varying locations asdescribed, for example, in the '581 application.

[0071] It should be noted that it is well within the skill of one havingordinary skill in the art to substitute different optical components formany of the specific optical components shown in FIG. 4. For example, avariety of different polarizing devices, beam splitters, collimatingoptics, lenses, SLMs and mirrors can be used in hologram recorder 400.Additionally, although FIG. 1 illustrates a system for producingreflection holograms, systems for producing transmission holograms usingthe devices and techniques described above can also be implemented.

[0072] As compared with hologram recorder 100 hologram recorder 400includes several components tailored to the task of producing HPOholograms. For example, BLD′ can be designed to be movable in at leastone direction. As illustrated in FIG. 4, BLD′ is designed to move in andout of the page. The band-limited diffuser is an optional diffractiveoptical element that redistributes the object beam across the entirehogel with a uniform intensity. If the diffuser is not used, the objectbeam would typically have a bright or “hot” spot along the centerline ofthe hogel. This would make the object-to-reference beam intensity ratiounbalanced and produce a brightness varying hogel. However, using aband-limited diffuser can produce an undesirable side effect. Thestructure on the diffuser is imaged by the Fourier-transform lens FCLand superimposed on hologram scene in the far field. If the hogel sizeis large, e.g., 1 mm square, the feature size of the diffuser structureis small and the side effect is not noticeable. However, if the hogelsize is small, the corresponding diffuser structure is more noticeableand produces a diffuser pattern on the hologram. An HPO recorder such asrecorder 400 typically has a 0.2 mm hogel width. A speckle type ofpattern associated with the band-limited diffuser has been observed ifthe hologram has a bright uniform background.

[0073] To minimize the speckle effect on the HPO hologram caused byusing band-limited diffusers, the diffuser is mounted on a movingmechanism to make the location of the diffuser vary from hogel to hogel.Consequently, each hogel has a unique far field speckle pattern. Thespatial mismatch of speckle patterns from hogel to hogel produces anaveraging effect when the hogels been viewed simultaneously. Due to therandom nature of the diffuser pattern and associated speckle patterns,the spatial averaging will make the hologram look uniform. Since therecorder is designed for use with pulsed lasers and thus mechanicalstability of the diffuser is not critical, diffuser BLD′ can be set inmotion continuously either by turning, oscillating, or translating.

[0074] As mentioned previously, the vertical diffuser VD increases thevertical view zone of an HPO hologram. Since the hologram recorder 400uses cylindrical lens FCL to produce an angular view zone along thehorizontal orientation of the hologram, the vertical orientation of lensFCL has no power. Thus in the vertical direction, the collimated objectbeam propagates without any divergence. If there were no diffuser VDinstalled in recorder 400, the hologram would show a verticallytruncated viewzone along a narrow horizontal line.

[0075] Because an HPO hologram does not contain information of parallaxalong the vertical orientation, the vertical view zone can be increasedsimply by inserting a one-dimensional diffuser or lenticular between thelens FCL and recording plate RP. However, this approach can present onedrawback. Since recording plate RP is generally transparent, thereference beam will transmit through the plate and strike the diffuser.The diffused reflected light will be sent back to recording plate RPaccompanied by the object beam resulting in a hologram that reconstructsboth the desired image and the diffusely reflected reference beam. Theviewer will see a holographic image with a bright vertical line on top.To overcome this problem, vertical diffuser VD can use a speciallydesigned holographic optical element (HOE) or a lenticular screencombined with an absorber-blocker film. The specialty HOE can bedesigned to diffract undesired light away from the holographic recordingmaterial. The lenticular screen serves as the diffusing medium and theabsorber-blocker film prevents the reference beam from reflecting backto the recording plate. Examples of these devices can be found in: (1)U.S. Pat. No. 6,369,920 entitled “Reference Beam Deflecting Element forRecording a Hologram,” naming Michael A. Klug as the inventor; and (2)U.S. patent application entitled “Reference Beam Absorber-Blockers,”Ser. No. ______, naming Michael A. Klug, Deanna McMillen, and QiangHuang as inventors, and filed on May 24, 2002 (Attorney Docket No.M-9362 US); both of which are hereby incorporated by reference herein inits entirety.

[0076] In general, using a lenticular screen as the vertical diffuser isadvantageous compared with other kinds of diffuser elements. Eachlenticule focuses the portion of object beam illuminating it to aspecific spot on the recording material without overlapping to theadjacent spots. Thus, interference or speckle noise caused by randomisotropic diffusers is reduced. Additionally, such a diffuser would makeit easier to produce interlaced hogels to increase hogel resolution onthe image plane. FIG. 5 illustrates an HPO hogel 500 formed by using alenticular screen diffuser for alternating recorded hologram portionsand “dead” spots. Because each lenticule produces a converging objectbeam and thus a well defined recorded portion. If an adjacent hogel isshifted vertically such that the hologram portions align to the “dead”spots of the previous hogel, interlacing hologram recording is achieved.As an example, fifteen interlaced HPO hogels are shown in FIG. 5.

[0077] As noted above, HPO recorder 400 utilizes a “ribbon” shapedreference beam that is generally incident on recording plate RP at a 45°angle with respect to the recording plate's surface normal. For eachpoint along the hogel, the beam paths should be closely matched inlength. Because of the aspect ratio of the beam, it can be difficult tosteer the beam with refractive and reflective optics to achieveobject-reference beam path matching on the recording plane. FIG. 6Ashows an example using a mirror to steer the beam. Even assuming thatthe center of the reference beam is matched with the object beam,extremes of the reference beam will still have optical paths that areeither too long or too short, as illustrated. To achieve path matchingfor the reference beam along the entire hogel, a diffraction grating isused to deflect the beam. As illustrated in FIG. 6B, a diffractiongrating GT is placed in the reference beam path so that all rays of thereference beam are incident to the grating perpendicularly. Thepropagation direction of the first order diffraction of the grating ischosen to be 45° with respect to the surface normal of the grating. Thedeflected first order diffraction beam is used as reference beam for thehologram recording at plate RP. This approach ensures that all raysreaching recording plate RP have an identical optical path length.Although FIG. 6B illustrates a transmissive diffraction grating GT,reflective gratings can also be used. Reflective gratings can be moreefficient than transmissive gratings.

[0078] Hologram recorders such as recorders 100 and 400 are designed toproduce holograms and particularly holographic stereograms at a highrate of speed. In order to accomplish this task, it is very importantthat the loading of a computer graphics image on spatial light modulatorSLM, the positioning of the holographic recording material at recordingplate RP, and the triggering of pulsed laser 110/410 be synchronized. Inone example of hologram recorder 100 the recorder is designed to record1 mm by 1 mm hogels using a unique image for each hogel at a rate of 60Hz (i.e., 60 hogels per second). To achieve this goal, hologram recorder100 uses holographic recording material translating system 120 to exposeeach individual hogel in the recording medium by making one pass througha row of hogels using one continuous motion. Hologram recorder 100exposes a single row using multiple 1 mm exposures. Upon completion of arow, the recording medium is indexed 1 mm vertically and the recordingcycle is repeated. Operating such a recording loop requires carefulattention to three separate synchronization tasks: (1) translating theholographic recording material at a 60 mm/sec rate; (2) providing theappropriate image data for each hogel recorded; and (3) providing alaser pulse only after the first two tasks are complete.

[0079]FIG. 7 shows a graph of the motion profile of a linear translatorsuitable for positioning holographic recording material according to therequirements of the first synchronization task. To perform the task, alinear translator carrying a suitable holographic recording material isaccelerated to 60 mm/sec. This velocity is kept constant for theduration of the hogel exposures and then ramped down. The process isrepeated for every row of hogels. After the first row of hogels isexposed. subsequent rows of hogels are particularly sensitive to twocritical points in the motion of the linear translator, as illustratedin FIG. 7. These points correspond to the beginning and ending of theconstant velocity phase of the linear translator's motion. If these twopoints do not match for every row of hogels, a noticeable artifact atthe beginning and/or the end of each row can manifest itself asillustrated in FIGS. 8A and 8B. If every row of hogels begins at thesame position along the x-axis, no noticeable alignment artifacts occuras shown in FIG. 8A. But, as shown in FIG. 8B, if hogel exposure beginsat an incorrect position for a given row, misalignment artifacts becomeapparent. Thus, it is important to use position controllers possessingadequate position control, speed, and repeatability.

[0080] The second synchronization task of delivering a unique image tospatial light modulator SLM for every 1 mm presents additionalchallenges. Not only is it important that each image for each hogel in arow of hogels be properly displayed on the SLM during the exposure of arow of hogels, but it is also important that each subsequent row ofrecorded hogels line up with the previous row of recorded hogels. Eventhough each hogel might be exposed at the right location on therecording media, the content of that hogel, i.e. the image used torecord that hogel, may not be in phase with the positioning system. Thisproblem is illustrated in FIGS. 9A and 9B. If the positioning system isexposing hogels at the correct location and the images are arriving inphase for every row of hogels, then the recorded array will appearcorrect as shown in FIG. 9A. In this case, the image stream alternatesbetween gray and white images. However, even if the hogels were exposedat the correct locations, a mismatch in phase between the stream ofimages and hogel positions can produce hogels with images that do notline up with subsequent rows as shown in FIG. 9B.

[0081] To solve this problem, the positioning system uses informationabout the image stream. This is accomplished by tapping into thevertical (v-sync) and horizontal (h-sync) synchronization signalsprovided as part of standard video signals such as those conforming toVGA, SVGA, and XGA standards. The v-sync signal (which typically causesa display device to perform a vertical retrace) and the h-sync signal(which typically causes a display device to perform a horizontalretrace) are used to synchronize the position of holographic recordingmaterial translating system 120 with the display of images on spatiallight modulator SLM. By correlating the v-sync pulse with positioninformation used to control holographic recording material translatingsystem 120, a constant phase relationship on a hogel-by-hogel basis canbe achieved. This correlation is illustrated in FIG. 10. FIG. 10 showsthe overlap of a standard v-sync signal with the motion profile of alinear translator. This information can be sent to a positioningcontroller such that each v-sync pulse represents a unique position ofthe holographic recording material. The v-sync signal can also be usedto trigger the movement profile which ensures a constant phaserelationship between subsequent rows.

[0082] Typical position controllers used in hologram recorder 100 makethe correlation between external signals (e.g., v-sync pulses) andposition by redirecting the controller's internal time base. However,the v-sync information alone is typically not enough information toadequately control a linear translator because there is insufficientinformation to interpolate between pulses. The end result of suchoperation is very jerky movement with instantaneous spikes inacceleration. This problem can be solved by using the much higherfrequency h-sync signal in conjunction with the v-sync signal. Forexample, at a typical SLM resolution with a refresh rate of 60 Hz (i.e.,v-sync=60 Hz) the corresponding h-sync frequency is approximately 64kHz. This. higher frequency can give the position controller ample datapoints between each v-sync pulse thereby allowing for much smootherinterpolation between position control data points. FIG. 11 illustratesthe concept of time-base redirection. By replacing the standard timeaxis on the graph with the h-sync pulses, the position controller canhave the necessary information from the image stream to accuratelycoordinate position. In the example illustrated there are exactly 1066h-sync pulses between each v-sync pulse.

[0083] The third task, namely providing a laser pulse only after thefirst two tasks are complete requires a solution similar to that usedfor providing the appropriated image data for each hogel recorded. Eachlaser pulse must have information about the appropriate image in theimage stream. For example, if only one laser pulse is to be used torecord each hogel, then one pulse must be provided at each hogelposition and that one pulse should contain image information for asingle image. If multiple pulses are used for each hogel, then severalsuccessive pulses containing the same image information can be used atone hogel position. Controlling the activation of the pulsed laser isconveniently performed using a “set output when at position” featureavailable in many position controllers. Since a v-sync pulse can be usedto represent the top of an image frame, a laser pulse should be emittedjust prior to the v-sync pulse. This can be achieved by using h-syncpulses to control the placement of the laser pulse. Since there aretypically 1066 h-sync pulses between every two v-sync pulses, finecontrol of the exact timing of each laser pulse can be achieved bytriggering the pulsed laser off of an h-sync pulse having an appropriatedelay with respect to a v-sync pulse.

[0084] As noted above, image synchronization is a particularly importantaspect of the hologram recording process. The positioning systemstypically used in synchronized holographic recording materialtranslating system 120 can be relatively autonomous and do notnecessarily need to be coupled to the computer providing images to theSLM. However, in many cases tight integration of the two systems ispreferable. Thus, latencies associated with the various data buses usedby the computer can be a problem. For example, if the positioning systemis controlled by a controller operating on a PC's ISA bus, the speed,responsiveness, and age of ISA bus technology can be limiting factors.Other limiting but related factors include the manner in which thecomputer's operating system handles various data buses. In general,faster buses, such as the PCI bus, are therefore desirable. Anotherexample of a relatively low-latency bus is the joystick/mouse portcommon to most PCs.

[0085] Those having ordinary skill in the art will readily recognizethat other display related signals can be used to control bothholographic recording material translating system 120 and pulsed laser110. Moreover, the preceding examples have discussed 1 mm by 1 mmhogels, but those having ordinary skill in the art will readilyrecognize that a variety of differently sized and shaped hogels can berecorded using the methods and devices disclosed in this patentapplication.

[0086] In the example described above, the video source signals are madethe “master” and thus the motor controls and laser pulse are slaved tothe master signal source. Other display systems could be used allowingthe motor control or the laser be the master and the display system theslave. Due to the sophistication of graphics cards available today, itis typically easier to make the video source the master than to design anew graphics system.

[0087] In using stepper motors as part of the translation system,various techniques for synchronizing the motor to other signals can beemployed. For example, the added resolution associated with using dcservos provides a significant advantage over other techniques. Thus, avariety of phase-locked loop techniques can be utilized including analogor digital hardware, a combination of the two, or a complete softwareimplementation.

[0088] In a further refinement of these techniques, one could sample theneeded error signal once based on the phase error between the videosignal and the motor position and make one phase correction at thebeginning of motion using, for example, one of two techniques: avelocity correction within the motor controller or a phase control motorlinked to the drive system via a planetary gear system. The phasecontrol motor would allow an independent change in phase withoutaffecting the constant rate of the drive motor. There would be no homeposition for the phase motor and the phase correction could be made ineither direction depending on the shortest motor motion to achieve aphase match. In one implementation, a 1 mm pitch screw is used toproduce a hogel pitch of one millimeter. An encoder on the screw shaftwould provide an absolute position for each hogel. At the beginning ofthe move, the timing of the vertical synchronization pulse is comparedto the screw position once the main drive motor reached full speed. Themeasured phase difference is fed to the phase correction motor control.Synchronizing each line of hogels independently should be adequateassuming the motor and the video signal would stay in synchronizationfor the duration of one line of hogels. In another example, utilizingboth horizontal and vertical synchronization signals allows an evenhigher degree of precision in the motion control.

[0089] Many techniques for producing and delivering the computergraphics and digitized images needed for hologram and holographicstereogram production are well known. Nevertheless, a high-speedhologram recorder such as hologram recorder 100 or hologram recorder 400presents several unique challenges regarding image production anddelivery. For example, if all of the computer graphics images for use ina particular holographic stereogram have been collected or rendered,processed, and stored on an appropriate storage device, hologramrecorder 100 could simply read the images out in the appropriate orderand display those images on spatial light modulator SLM. However to takefull advantage of the high-speed nature of hologram recorder 100 it isdesirable to include a computer system associated with hologram recorder100 to support dynamic construction of full parallax andhorizontal-parallax-only hogels, to utilize 3D graphics hardware torender the image stream instead of standard 2D interfaces, to use 3Dtextured geometry to render the image stream, and to store images in anoptimal format on the 3D graphics hardware for fast display on spatiallight modulator SLM.

[0090] In a data preparation phase, the image resolution is set to theappropriate values for the particular images to be recorded. A bit-depthof 32 bits (e.g., 8-bits for each of three colors and an 8 bit alphachannel) ensures smooth gradients. Image dithering at 16 bits can alsobe used to prepare the image data using lower bit-depth. Next, imagesare loaded into memory (typically the RAM of the computer systemproviding the images or the RAM associated with the graphics hardwareused to provide the images to spatial light modulator SLM). Someinterpolation of the images is typically performed to match the imagedataset to the available memory and recorder metrics. In the case ofhorizontal-parallax-only holographic stereograms, the images aretypically transformed and stored in graphics hardware memory; no furtherprocessing is required.

[0091] During a recording phase, system synchronization and hogelrecording are performed. To begin the process of recording a row ofhogels, a line start flag is sent to the position controller hardwareassociated with synchronized holographic recording material translatingsystem 120. This signals hologram recorder 100 that processing of theimage set for the next row of hogels is complete and that the computersystem is waiting for the hologram recorder to record the next line ofhogels. A line wait flag is polled on the position controller hardware.This flag is used to start the recorder and the rendering of images onnext v-sync signal. Next, the computer system provides image data forone hogel on each v-sync signal. These images for these hogels aretypically stored on computer graphics hardware and rendered by thehardware as discussed below. A column completion routine in the softwareexecuting on the computer system is next used to adjust relevantrecorder values and increment the column counter for the next column ofhogels. In the next stage, a waiting for recorder flag is polled. Duringthis stage, the next column of hogels is generated for the full parallaxholographic stereograms. A typical horizontal-parallax-only holographicstereogram will have only one row of hogels. When the column iscomplete, the recorder is checked to determine if the waitingforrecorder flag is set before continuing on to the next column of hogels.

[0092] During a rendering phase typically proceeding a recording phase,operation can differ depending on the recording ofhorizontal-parallax-only holographic stereograms or holographicstereograms with both horizontal and vertical parallax (full parallax).The rendering phase can include the reparameterization of image data asdescribed, for example, in U.S. patent application Ser. No. 10/036,814,entitled “Efficient Block Transform Including Pre-processing and PostProcessing for Autostereoscopic Displays,” filed Oct. 19, 2001, andnaming Emilio Camahort, Mark E. Holzbach and Robert L. Sitton as theinventors, and which is hereby incorporated by reference herein in itsentirety. Horizontal-parallax-only rendering uses a single row ofhorizontal directional views. The reparameterization of the single rowof horizontal directional views typically results in a set of hogelimages each having only one row of pixels. The row of pixels istypically scaled as necessary to fill the spatial light modulator of thehologram recorder. For full parallax holographic stereograms, therendering phase typically displays entire hogels on the spatial lightmodulator scaled as necessary. In some examples of the rendering phase,reparameterization is performed in real time. Thus, the softwareassembling the hogel views specifically selects the appropriate point oflight from the image data set (i.e., the source images) and places it atthe appropriate location within the hogel view. If the desired point oflight from the image data set exists between two hogel views,sub-sampling can be performed. If the desired point of light from theimage data set does not exist (e.g., the data is missing or the sourceimage resolution is too low) an average of data points can be performed.

[0093] The various phases of producing image data for the hologramrecorder and providing that data to the hologram recorder are typicallyperformed by specialized computer software executing on one or morecomputer systems. Those having ordinary skill in the art will readilyrecognize that the techniques and methods discussed above can beimplemented in software using a variety of computer languages,including, for example, traditional computer languages such as assemblylanguage, Pascal, and C; object oriented languages such as C++ and Java;and scripting languages such as Perl and Tcl/Tk. Additionally, thesoftware can be provided to the computer system via a variety ofcomputer readable media including electronic media (e.g., flash memory),magnetic storage media (e.g., hard disk drives, a floppy disk, etc.),optical storage media (e.g., CD-ROMs), and communications mediaconveying signals encoding the instructions (e.g., via a network coupledto a network interface in the computer system). The computer system orsystems typically used include devices such as a keyboard, a mouse, anetwork interface, a graphics & display hardware, a hard disk drive, anda CD-ROM drive, all of which are coupled to a processor by acommunications bus. It will be apparent to those having ordinary skillin the art that such computer system can also include numerous elementsnot described, such as additional storage devices, communicationsdevices, input devices, and output devices.

[0094] Those having ordinary skill in the art will readily recognizethat a variety of different types of optical components can be used inplace of the components discussed above. Moreover, the description ofthe invention set forth herein is illustrative and is not intended tolimit the scope of the invention as set forth in the following claims.Variations and modifications of the embodiments disclosed herein may bemade based on the description set forth herein, without departing fromthe scope and spirit of the invention as set forth in the followingclaims.

1. A method of recording holograms comprising: providing a holographicrecording material; pre-sensitizing the holographic recording materialby exposing the holographic recording material to an incoherentbroadband light source; and exposing the holographic recording materialto an interference patterned formed by a reference beam from a pulsedlaser and an object beam from the pulsed laser.
 2. The method of claim 1wherein the exposing the holographic recording material furthercomprises: orienting at least one of the reference beam and the objectbeam at an oblique angle with respect to the holographic recordingmaterial.
 3. The method of claim 2 wherein the orienting at least one ofthe reference and the object beam further comprises: positioning the atleast one of the reference beam and the object beam at an angle ofapproximately 45° with respect to a surface normal of the holographicrecording material.
 4. The method of claim 1 wherein the holographicrecording material is one of a photopolymer, a dichromated gelatin, anda silver halide emulsion.
 5. The method of claim 4 wherein theholographic recording material is a photopolymer laminated to asubstrate.
 6. The method of claim 1 wherein the incoherent broadbandlight source includes at least one of: a halogen light source,florescent light source, an incandescent light source, a white-lightlight emitting diode (LED) light source, and a plurality of narrowbandincoherent light sources.
 7. The method of claim 1 further comprising:adjusting the path length of at least one of the reference beam and theobject beam so that the reference beam and the object beam are coherentwith respect to each other.
 8. The method of claim 7 wherein theadjusting the path length further comprises: providing a firststationary mirror operable to reflect the at least one of the referencebeam and the object beam; providing at least one movable mirror operableto reflect light from the first stationary mirror to a second stationarymirror; and adjusting a position of the at least one movable mirror. 9.The method of claim 1 further comprising: intensity modulating theobject beam with a spatial light modulator.
 10. The method of claim 1further comprising: waiting a predetermined period of time between thepre-sensitizing the holographic recording material and the exposing theholographic recording material.
 11. The method of claim 1 wherein theexposing the holographic recording material to an incoherent broadbandlight source further comprises: exposing only a small portion of theholographic recording material to the incoherent broadband light source.12. The method of claim 1 wherein the exposing the holographic recordingmaterial to an incoherent broadband light source further comprises:exposing substantially all of the holographic recording material to theincoherent broadband light source. 13-22. (Cancelled)
 23. A method ofrecording holograms comprising: providing a holographic recordingmaterial; pre-sensitizing the holographic recording material by exposingthe holographic recording material to at least one of an incoherentbroadband light source and a plurality of laser pulses; orienting atleast one of a reference beam from a pulsed laser and an object beamfrom the pulsed laser at an oblique angle with respect to theholographic recording material; exposing the holographic recordingmaterial to an interference pattern formed by the reference beam and theobject beam.
 24. The method of claim 23 wherein the orienting at leastone of the reference and the object beam further comprises: positioningthe at least one of the reference beam and the object beam at an angleof approximately 45° with respect to a surface normal of the holographicrecording material.
 25. The method of claim 23 wherein the holographicrecording material is one of a photopolymer, a dichromated gelatin, anda silver halide emulsion.
 26. The method of claim 25 wherein theholographic recording material is a photopolymer laminated to asubstrate.
 27. The method of claim 23 wherein the incoherent broadbandlight source includes at least one of: a halogen light source,florescent light source, an incandescent light source, a white-lightlight emitting diode (LED) light source, and a plurality of narrowbandincoherent light sources.
 28. The method of claim 23 further comprising:adjusting the path length of at least one of the reference beam and theobject beam so that the reference beam and the object beam are coherentwith respect to each other.
 29. The method of claim 28 wherein theadjusting the path length further comprises: providing a firststationary mirror operable to reflect the at least one of the referencebeam and the object beam; providing at least one movable mirror operableto reflect light from the first stationary mirror to a second stationarymirror; and adjusting a position of the at least one movable mirror. 30.The method of claim 23 further comprising: intensity modulating theobject beam with a spatial light modulator.
 31. The method of claim 23further comprising: waiting a predetermined period of time between thepre-sensitizing the holographic recording material and the exposing theholographic recording material.
 32. An apparatus for recordingholographic stereograms, comprising: a pulsed light source that producesa coherent beam; a material holder holding a pre-sensitized holographicrecording material, the pre-sensitized holographic recording materialbeing pre-sensitized by exposing the holographic recording material toat least one of an incoherent broadband light source and a plurality oflaser pulses; and an optical system operable to direct at least aportion of the coherent beam to the holographic recording material. 33.The apparatus of claim 32 wherein optical system further comprises: abeam splitter that splits the coherent beam into an object beam and areference beam; an object beam optical system for directing the objectbeam to interfere with the reference beam at the pre-sensitizedholographic recording material; and a reference beam optical system fordirecting the reference beam to interfere with the object beam at thepre-sensitized holographic recording material.
 34. The apparatus ofclaim 33 wherein at least one of the reference beam optical system andthe object beam optical system includes an optical pathlength matchingdevice comprising: a first stationary mirror positioned to reflect atleast one of the reference beam and the object beam; at least onemovable mirror positioned to receive the at least one of the referencebeam and the object beam from the first stationary mirror and reflectthe at least one of the reference beam and the object beam; and a secondstationary mirror positioned to receive the at least one of thereference beam and the object beam from the at least one movable mirror;wherein the optical pathlength of the at least one of the reference beamand the object beam is adjusted by moving the at least one movablemirror.
 35. The apparatus of claim 33 wherein the object beam opticalsystem includes a spatial light modulator for intensity modulating theobject beam.
 36. The apparatus of claim 35 further comprising: acomputer coupled to the spatial light modulator and programmed tocontrol delivery of a rendered image to the spatial light modulator. 37.The apparatus of claim 36 further comprising: a material holdertranslation system operable to position the material holder holding thepre-sensitized holographic recording material.
 38. The apparatus ofclaim 37 wherein the computer is coupled to the material holdertranslation system and is further programmed to synchronize positioningof the material holder translation system with delivery of the renderedimage to the spatial light modulator.
 39. The apparatus of claim 38wherein: a velocity of the material holder translation system is keptconstant for the duration of one row of hogel exposures.
 40. Theapparatus of claim 38 wherein: the computer is further programmed tosynchronize positioning of the material holder translation system withat least one of a vertical synchronization signal and a horizontalsynchronization signal provided with the rendered image to the spatiallight modulator.
 41. The apparatus of claim 36 wherein the pulsed lightsource is triggered based on at least one of a vertical synchronizationsignal and a horizontal synchronization signal provided with therendered image to the spatial light modulator.
 42. The apparatus ofclaim 33 further comprising: at least one of a transmissive diffractiongrating and a reflective diffraction grating positioned to diffract thereference beam so as to enhance path matching for the reference beamwhen the reference beam strikes the recording material.
 43. Theapparatus of claim 32 further comprising: an incoherent broadband lightsource positioned to pre-sensitize the holographic recording material.44. The apparatus of claim 43 wherein the incoherent broadband lightsource includes at least one of: a halogen light source, florescentlight source, an incandescent light source, a white-light light emittingdiode (LED) light source, and a plurality of narrowband incoherent lightsources.
 45. The apparatus of claim 43 wherein the incoherent broadbandlight source is further positioned to only expose a small portion of theholographic recording material.
 46. The apparatus of claim 32 whereinthe at least a portion of the coherent beam is oriented at an obliqueangle with respect to the holographic recording material.
 47. Theapparatus of claim 46 wherein the at least a portion of the coherentbeam is oriented at an angle of approximately 45° with respect to asurface normal of the holographic recording material.
 48. The apparatusclaim 32 wherein the holographic recording material is one of aphotopolymer, a dichromated gelatin, and a silver halide emulsion. 49.The apparatus of claim 48 wherein the holographic recording material isa photopolymer laminated to a substrate.
 50. The apparatus of claim 32wherein the plurality of laser pulses are provided by the at least aportion of the coherent beam.